The invention relates to an electric machine with a rotor which can rotate about a rotation axis and has a winding to be cooled to a low temperature, in particular a superconducting winding, which is surrounded by a securing means with a wrap. A rotor for a corresponding machine is disclosed in DE 199 43 783 A1.
Two-pole or multi-pole windings of electric machines can be arranged uniformly on the exterior circumference of a winding support or a so-called pole core about a common axis, with a DC current having alternating current directions flowing through the windings. These windings can be applied to the outside of the winding support or arranged in slot-like recesses.
A corresponding electric machine with this type of multi-pole winding construction is disclosed in EP 0 805 545 A1. Each individual pole of this machine is produced by a partial winding of the “racetrack” type, with the superconducting conductors being wound about an yoke and located in a dedicated cryostat. A suitable superconducting material for the conductor is, in particular, Nb3Sn.
Metal oxide superconducting materials, which have transition temperatures above 77 K and which are therefore also referred to as High-Tc superconductors or HTS materials, are known since 1987. Accordingly, attempts have been made to also prepare windings with these conductors. However, it has been observed that the HTS conductors known to date can only support a relatively low current density in magnetic fields having an induction in the range of one Tesla or more, where electric machines operate. For this reason, windings made from these conductors must be maintained at a temperature below 77 K in spite of the relatively high transition temperatures of their conductors.
The aforedescribed published patent application DE 199 43 783 A1 describes a rotor with a multi-pole winding that uses HTS conductors. This rotor has a winding support with coil sections which are arranged with a relative 90° offset in the circumferential direction and form the four individual poles of the rotor. Each of the coil sections consists of a stack of planar coil elements of the “racetrack” type, wherein each coil element is prepared from HTS superconductor tape. The coil sections are arranged so that their outside contours match at least substantially a common exterior cylindrical surface of the support or pole core.
Advantageously, corresponding HTS rotor windings are produced by first prefabricating and then testing the coils. These coils or coil packets can be combined into larger units and subsequently mounted on the pores of the supporting rotor body. During operation, the coils are subjected to considerable centrifugal forces caused by the rotation as well as to magnetic forces. These forces attempt to pull the coils toward the outside. Because a movement of the winding and in particular of the superconducting material is undesirable, the coils have to be properly secured. A suitable securing means can be a tubular sheath or a wrap made from glass fiber-reinforced plastic material. However, with large forces, the strength and stiffness of the fiber-reinforced plastic material is generally insufficient, so that typically tubular sheaths (see U.S. Pat. No. 4,060,743 A). Alternatively, segmented tubular sheaths or rings (see DE 32 12 416 A1) made of metal are used as securing means, because the elastic modulus of metals is typically 5 to 10 times greater than those of fiber-reinforced plastic. For example, one disclosed metal jacket is formed by winding around the coil support a steel tape analogous to a fiber tape (see WO 00/49703 A). Because a metal tape, unlike a fiber tape, has a high transverse stiffness already during the winding process, such metal wrap is difficult to manufacture.